Endothelial cells in culture: an experimental model for the study of vascular dysfunctions

Culture of endothelial cells started two decades ago and is now a useful tool in understanding endothelial physiology and the study of the interaction of endothelial cells with blood cells and various mediators. In vitro proliferation can be measured by [3H]thymidine incorporation in defined conditions and gives reproducible results. Endothelial cells can be activated by several stimuli, including cytokines such as tumor necrosis factor-alpha and interleukin-1. Part of endothelial cell activation is defined by expression or overexpression of leukocyte adhesion molecules. Intracellular adhesion molecule (ICAM), E-selection and vascular adhesion molecule (VCAM) are receptor molecules for leukocyte adhesion. Leukocyte adhesion to endothelium can be measured in static but also in rheologically defined flow conditions. Normal red blood cells (RBCs) do not adhere to endothelium, while RBC from patients with sickle cell anemia, diabetes mellitus, and malaria have an increased adhesion to endothelium which is mediated by specific VCAM, receptor for advanced glycated end-products (RAGE), and ICAM, respectively. Binding of blood cells or activation by cytokine is followed by a series of reactions in endothelial cells associated with the modulation of prostacyclin, nitric oxide, tissue factor, and cytokine production. Modification of endothelial cell functions in culture is correlated to in vivo alteration of vascular wall properties, further supporting these cells in culture as a relevant experimental model.     

Ionizing radiation mediates expression of cell adhesion molecules in distinct histological patterns within the lung

Inflammatory cell infiltration of the lung is a predominant histopathological change that occurs during radiation pneumonitis. Emigration of inflammatory cells from the circulation requires the interaction between cell adhesion molecules on the vascular endothelium and molecules on the surface of leukocytes. We studied the immunohistochemical pattern of expression of cell adhesion molecules in lungs from mice treated with thoracic irradiation. After X-irradiation, the endothelial leukocyte adhesion molecule 1 (ELAM-1; E-selectin) was primarily expressed in the pulmonary endothelium of larger vessels and minimally in the microvascular endothelium. Conversely, the intercellular adhesion molecule 1 (ICAM-1; CD54) was expressed in the pulmonary capillary endothelium and minimally in the endothelium of larger vessels. Radiation-mediated E-selectin expression was first observed at 6 h, whereas ICAM-1 expression initially increased at 24 h after irradiation. ICAM-1 and E-selectin expression persisted for several days. P-selectin is constitutively expressed in Weibel-Palade bodies in the endothelium, which moved to the vascular lumen within 30 min after irradiation. P-selectin was not detected in the pulmonary endothelium at 6 h after irradiation. The radiation dose required for increased cell adhesion molecule expression within the pulmonary vascular endothelium was 2 Gy, and expression increased in a dose-dependent manner. These data demonstrate that ICAM-1 and E-selectin expression is increased in the pulmonary endothelium following thoracic irradiation. The pattern of expression of E-selectin, P-selectin, and ICAM-1 is distinct from one another.     

Expression of the beta 7 integrin by human endothelial cells

Integrin adhesion receptors mediate fundamental intercellular interactions of many cell types as well as cellular interactions with specific extracellular matrix molecules. To date, the beta 7 integrin has been shown to be expressed by leukocyte subsets and to mediate interactions of these cells with extracellular matrix molecules as well as with endothelial and epithelial cells. The data presented here indicate that human endothelial cells also express the beta 7 integrin both in vitro and in situ. Analysis of cDNA indicated that endothelial beta 7 was identical to that expressed by leukocytes. Cell surface expression of beta 7 was increased by exposure of the endothelium to the pro-inflammatory cytokines, tumor necrosis factor-alpha and interleukin-1 beta. In leukocytes, beta 7 complexes with alpha 4 or alpha E integrin chains. Endothelial cells also expressed a number of alpha-integrin chains, including alpha 4, but not alpha E. The expression and utilization of beta 7, presumably complexed with alpha 4, by endothelial cells may be instrumental in the maintenance of the function or phenotype of endothelial cells.     

The white blood cell adhesion molecule E-selectin predicts restenosis in patients with intermittent claudication undergoing percutaneous transluminal angioplasty

BACKGROUND: Experimental studies have shown that endothelial dysfunction is an early event preceding restenosis. Monocytes and neutrophils have been shown to bind to damaged endothelium via the cell adhesion molecules (CAMs). The selectins are involved in capturing the leukocytes and tethering them to the endothelium. E-selectin is a CAM that is only expressed on activated endothelial cells. Its ligands are expressed on monocytes and neutrophils and it has been found to exist in a soluble form. This soluble form may represent a marker for endothelial damage and may be a precursor of smooth muscle proliferation. METHODS AND RESULTS: Fifty-four patients who were undergoing peripheral arterial balloon angioplasty had blood sampled before angioplasty. E-selectin was measured in plasma with the use of an ELISA. At follow-up angiogram, 30% (n=14) of the patients had restenosed at 1 year. There was a significant difference in baseline E-selectin levels in patients who restenosed compared with those who did not (65.3 ng/mL [58.25 to 78.05] versus 52.3 [34.2 to 62.1], Mann-Whitney U, P<.007). Endothelial activation with subsequent adherence of white blood cells is an important step in restenosis. CONCLUSIONS: We have shown an increased level of shed E-selectin in patients destined for restenosis and suggest that this work further supports a role for white blood cell/endothelial interaction in restenosis after angioplasty.     

L-selectin crosslinking induces integrin-dependent adhesion: evidence for a signaling pathway involving PTK but not PKC

L-selectin mediates the initial contact of leukocytes with the endothelium prior to extravasation. Here we demonstrate that L-selectin engagement can induce rapid and avid integrin-dependent T cell adhesion to recombinant immobilized cell adhesion molecules (CAMs) including ICAM-1, ICAM-3, and VCAM-1, as well as to the extracellular matrix protein fibronectin (FN). L-selectin-induced adhesion to these integrin ligands shares characteristics with CD3 mAb- or phorbol ester-induced adhesion in requiring metabolic energy, tyrosine kinase and ligand-stimulated Ca2+ channel activity. However, L-selectin-induced adhesion is distinct from that induced by phorbol ester or CD3 crosslinking in being relatively independent of protein kinase C (PKC) activity and actin polymerization. Consistent with the higher levels of L-selectin expression on CD45RA+ (naive) cells, L-selectin crosslinking induces a greater proportion of naive relative to memory cell binding to CAMs and FN. In contrast, exposure to phorbol ester or CD3 crosslinking is more effective in inducing CD45RO+ (memory) cell adhesion. Thus, in addition to its role in leukocyte capture and rolling on the endothelium, L-selectin may contribute to beta 1 and beta 2 integrin-dependent binding and arrest.     

Adhesion molecules in clinical medicine

Cellular adhesion molecules (CAMs) are critical components in the processes of embryogenesis, tissue repair and organization, lymphocyte function, lymphocyte homing and tumor metastasis, as well as being central to the interactions between hemopoietic progenitors and bone marrow microenvironment, and between leukocytes and platelets with vascular endothelium. Expression of CAMs regulates normal hemopoiesis and migration and function of mature hemopoietic cells. CAMs are an important part of the inflammatory response and may regulate cytokine synthesis. In addition, CAM expression may be critical for tumorigenesis. Monoclonal antibodies to CAMs have been developed for clinical use; initial results suggest that these agents have great potential in the prevention and treatment of inflammation, thrombosis, reperfusion injury, and graft rejection.     

Cigarette smoking is associated with increased human monocyte adhesion to endothelial cells: reversibility with oral L-arginine but not vitamin C

OBJECTIVES: This study sought to assess the effect of cigarette smoking on adhesion of human monocytes to human endothelial cells and to measure the effect of L-arginine and vitamin C supplementation on this interaction. BACKGROUND: Cigarette smoking has been associated with abnormal endothelial function and increased leukocyte adhesion to endothelium, both key early events in atherogenesis. Supplementation with both oral L-arginine (the physiologic substrate for nitric oxide) and vitamin C (an aqueous phase antioxidant) may improve endothelial function; however, their benefit in cigarette smokers is not known. METHODS: Serum was collected from eight smokers (mean [+/-SD] age 33 +/- 5 years) with no other coronary risk factors and eight age- and gender-matched lifelong nonsmokers. The serum was added to confluent monolayers of human umbilical vein endothelial cells and incubated for 24 h. Human monocytes obtained by counterflow centrifugation elutriation were then added to these monolayers for 1 h, and adhesion then was measured by light microscopy. To assess reversibility, monocyte/ endothelial cell adhesion was then measured for each subject 2 h after 2 g of oral vitamin C and 2 h after 7 g of oral L-arginine. RESULTS: In smokers compared with control subjects, monocyte/ endothelial cell adhesion was increased (46.4 +/- 4.5% vs. 27.0 +/- 5.2%, p < 0.001), endothelial expression of intercellular adhesion molecule (ICAM)-1 was increased (0.31 +/- 0.02 vs. 0.22 +/- 0.03, p = 0.004), and vitamin C levels were reduced (33.7 +/- 24.1 vs. 53.4 +/- 11.5 mumol/liter, p = 0.028). After oral L-arginine, monocyte/ endothelial cell adhesion was reduced in smokers (from 46.4 +/- 4.5% to 35.1 +/- 4.0%, p = 0.002), as was endothelial cell expression of ICAM-1 (from 0.31 +/- 0.02 to 0.27 +/- 0.01, p = 0.001). After vitamin C, there was no significant change in monocyte/ endothelial cell adhesion or ICAM-1 expression from baseline in the smokers despite an increase in vitamin C levels (to 115 +/- 7 mumol/liter). CONCLUSIONS: Cigarette smoking is associated with increased monocyte-endothelial cell adhesion when endothelial cells are exposed to serum from healthy young adults. This abnormality is acutely reversible by oral L-arginine but not by vitamin C.     

Expression of ICAM-1, VCAM-1, and LFA-1 in adenocarcinoma of the lung with observations on the expression of these adhesion molecules in non-neoplastic lung tissue

Intercellular adhesion molecule (ICAM-1), vascular cell adhesion molecule (VCAM-1), and the lymphocyte function-associated antigen (LFA-1) are cell adhesion molecules thought to play an important role in the complex process of airway inflammation and tumor cell growth. The aim of this study was to examine the distribution of ICAM-1, VCAM-1, and LFA-1 in adenocarcinoma of lung and in major cellular compartments of non-neoplastic lung tissue. We examined cellular compartments in tissue from five bronchoalveolar carcinomas, three acinar adenocarcinomas, and one colon cancer metastatic to the lung. The compartments in neoplasms included the tumor cells proper, endothelial cells within the tumor vasculature, tumor stromal cells, and tumor-infiltrating lymphocytes. The compartments in non-neoplastic lung tissue included lung endothelial cells, pulmonary lymphocytes, interstitial fibroblasts, Type II alveolar pneumocytes, and bronchial epithelial cells. ICAM-1 was expressed in tumor cells from all of the nine adenocarcinomas. In contrast, VCAM-1 expression was not identified in tumor cells from any of the nine adenocarcinomas. ICAM-1 was expressed in all cellular compartments of the non-neoplastic lung tissue, whereas VCAM-1 was expressed only in pulmonary lymphocytes and interstitial fibroblastic cells. LFA-1 was uniformly expressed in tumor-infiltrating lymphocytes from each of the nine tumors and all of the lymphocytes in non-neoplastic lung tissue. This study showed major differences in the expression of ICAM-1 and VCAM-1 in tumor cells from pulmonary adenocarcinoma and also provided evidence for a wider distribution of ICAM-1, compared with VCAM-1, in non-neoplastic cellular compartments of the lung. ICAM-1 expression was particularly noticeable in bronchial and alveolar epithelial cells. Upregulation of ICAM-1 in pulmonary adenocarcinoma might foster binding by LFA-1-bearing lymphocytes, with a possible impact on the vulnerability of tumor cells to host defense mechanisms.     

Deficiency of inflammatory cell adhesion molecules protects against atherosclerosis in mice

Leukocyte and endothelial cell adhesion molecules (CAMs) are essential for emigration of leukocytes, with the selectins mediating the initial step of leukocyte "rolling" and the beta 2-(CD18) integrins and intercellular adhesion molecule-1 (ICAM-1) being important for firm adhesion and emigration. On the basis of evidence for an inflammatory component in the pathogenesis of atherosclerosis, including increased expression of CAMs, cytokines, and growth factors, we tested the hypothesis that decreased expression of inflammatory CAMs would reduce susceptibility to atherosclerosis. Using C57BL/6 mice fed a high-fat diet, we observed a 50% to 75% reduction in atherosclerotic fatty streaks in mice with homozygous mutations for ICAM-1, P-selectin, CD18, both ICAM-1 and CD18, or both ICAM-1 and P-selectin. In contrast to previous evidence of increased expression of CAMs in atherosclerotic lesions, which does not prove a cause-and-effect relationship, these data indicate directly that the level of expression of CAMs can determine the susceptibility to the formation of atherosclerotic fatty streaks. The results suggest that genetic variation at these loci could influence susceptibility to atherosclerosis and that pharmacological reduction of the expression or function of these CAMs might protect against atherosclerosis.     

Soluble E-selectin enhances intercellular adhesion molecule-1 (ICAM-1) expression in human tumor cell lines

E-selectin mediates neovascularization via its soluble form, while its membrane-bound form initiates binding of tumor cells to vascular endothelium. Therefore, it was studied whether soluble E-selectin regulates further adhesion molecules on tumor cells. In tumor cells but not in related nonmalignant cells, intercellular adhesion molecule (ICAM)-1 expression was strikingly increased from 5 to 68% positive cells by in vitro inoculation of a recombinant E-selectin-IgG1 within 24 h, as analyzed by flow cytometry. The absence of changes in the expression of vascular cell adhesion molecule, integrin ligands (CD11a, CD18, integrin alpha 4), and sialyl-Lewis X indicates a specific effect of soluble E-selectin on ICAM-1. A cell adhesion assay revealed that the enhanced adhesion on T-cells to tumor cells mediated by soluble E-selectin-induced ICAM-1 expression was at a maximum after a 12-h incubation period. Therefore, ICAM-1 regulation on tumor cells might be a mechanism of immune escape.     

Kinetics of adhesion molecule expression and spatial organization using targeted sampling fluorometry

Cellular interactions with the vascular wall under flow conditions are controlled, in part, by the density of adhesion molecules on endothelial cells. The spatial arrangement and absolute levels of these molecules over the endothelium are therefore important determinants of cellular localization. Many biochemical and functional studies have characterized the interactions between leukocytes and endothelial monolayers, but no reliable method has been reported for quantifying the spatial expression of adhesion molecules on intact endothelial cell monolayers. We report the development of targeted sampling fluorometry (TSF), which uses standard immunostaining, fluorescence microscopy and digital image analysis techniques to analyze cell surface molecule expression on a cell-by-cell basis. This technique is performed on an intact monolayer and results in cellular intensity distributions that reflect spatial heterogeneity in adhesion molecule expression. We demonstrate the use of targeted sampling fluorometry in a study of the kinetics of tumor necrosis factor alpha-induced activation of human umbilical vein endothelial cell monolayers and show that the spatial patterns of adhesion molecule expression correlate with the locations of bound lymphocytes.     

Reduced intracellular oxidative metabolism promotes firm adhesion of human polymorphonuclear leukocytes to vascular endothelium under flow conditions

The interaction of polymorphonuclear leukocytes (PMN) with the vascular endothelium and their subsequent extravasation to the tissues is a key step during different physiological and pathological processes. In certain of these pathologies the oxygen tension becomes very low, leading to reduced cellular oxidative status. To evaluate the effect of lowering the intracellular redox status in the interaction of PMN with the endothelium, exposure to hypoxic conditions as well as treatment with different antioxidant agents was carried out. PMN exposure to hypoxia enhanced beta2 integrin-dependent adhesion to intercellular adhesion molecule-1-coated surfaces, concomitant with a decrease in the intracellular redox status of the cell. As occurs with hypoxia, treatment with antioxidants produced a decrease in the oxidation state of PMN. These agents enhanced adhesion of PMN to human umbilical vein endothelial cells stimulated with tumor necrosis factor-alpha (TNF-alpha), and this effect was also mediated by beta2 integrins LFA-1 and Mac-1. Adhesion studies under defined laminar flow conditions showed that the antioxidant treatment induced an enhanced adhesion mediated by beta2 integrins with a decrease in the fraction of PMN rolling on TNF-alpha-activated endothelial cells. The up-regulated PMN adhesion was correlated to an increase in the expression and activation of integrin Mac-1, without loss of L-selectin surface expression. Altogether, these results demonstrate that a reduction in the intracellular oxidative state produces an enhanced beta2 integrin-dependent adhesion of PMN to stimulated endothelial cells under conditions of flow.     

ICAM-1 upregulation in distant tissues after hepatic ischemia/reperfusion: a clue to the mechanism of multiple organ failure

BACKGROUND/PURPOSE: Endothelial cell adhesion molecules (ECAMs) are felt to play an important role in ischemia/reperfusion (I/R) injury by causing adhesion of leukocytes to endothelial cells. It is possible that ECAMs play a role in multiple organ system failure. ICAM-1 is one of the adhesion molecules that has been shown to be upregulated in response to cytokines. This upregulation leads to leukocyte endothelial cell interaction (adhesion) and to neutrophil infiltration of the affected tissue. The purpose of our study was to measure ICAM-1 expression in the liver and other organs after hepatic ischemia/reperfusion (I/R). METHODS: A laparotomy was performed on 14 Sprague-Dawley rats; 45 minutes of occlusive ischemia to the left lateral lobe was followed by 5 hours of reperfusion. The rat was injected with I125-labeled ICAM-1 MAb and I131-labeled nonbinding MAb (to control for nonspecific accumulation of ICAM-1 MAb). Entire organs were harvested and accumulated activity was measured in each organ. ICAM-1 levels were expressed as percent injected dose per gram of tissue. Control animals underwent sham laparotomy. RESULTS: ICAM-1 was upregulated in the ischemic lobe of the liver, nonischemic lobe of the liver, heart, kidney, intestine, and pancreas. Up-regulation in the lung was not significant. Both the lung and liver had high constitutive levels of ICAM-1. CONCLUSIONS: These data show that (1) significant hepatic upregulation of ICAM-1 after hepatic ischemia/reperfusion and (2) significant ICAM-1 upregulation in other tissues (heart, kidney, and intestine) after hepatic ischemia/reperfusion. The ICAM-1 upregulation in distant organs is likely mediated by cytokines such as tumor necrosis factor (TNF). These data show that leukocyte endothelial cell interactions in distant organs may be mediated by hepatic ischemia/reperfusion. This is a possible explanation for how failure of one organ can lead to failure of others in multiple organ system failure.     

Intimal hyperplasia after balloon injury is attenuated by blocking selectins

BACKGROUND: Cell adhesion molecules facilitate the adherence of platelets and leukocytes to the vascular endothelium in response to injury. Restenosis after balloon angioplasty is thought to represent the response to vascular injury. The role of cell adhesion in this process is unclear. METHODS AND RESULTS: This study was performed in New Zealand White rabbits that underwent balloon angioplasty of the iliac artery. Expression of the cell adhesion molecule E-selectin on endothelium was determined by immunohistochemistry and increased at 6 hours with a peak expression 24 to 48 hours after balloon injury, returning to baseline by 1 week. The expression of L-selectin on circulating leukocytes, measured by flow cytometry, was significantly increased at 48 hours, with return to baseline by 1 week. In seven animals, the selectins were blocked with an analogue of sialyl-Lewis(x) given as an I.V. bolus of 10 mg/kg followed by 2 mg x kg(-1) x h(-1) I.P. infusion for 7 days. After 4 weeks, compared with control animals, the study group had a larger lumen area (57.7 versus 44.7 mm2, P<.05), smaller intima area (9.0 versus 19.2 mm2, P<.01), smaller intima/media ratio (0.4 versus 1.0, P<.01), and a smaller percent area stenosis (15.6% versus 34.3%, P<.01). CONCLUSIONS: The cell adhesion molecules E-selectin and L-selectin are expressed after balloon injury. Blockade of the selectins has a favorable effect on the response to vascular injury.     

Induction of P-selectin by oxidized lipoproteins. Separate effects on synthesis and surface expression

Leukocyte binding to the endothelium is one of the earliest events in the occurrence of atherosclerosis. Leukocyte adhesion molecules involved in this process have not been definitely identified. We have found that treatment of human aortic endothelial cells (HAECs) with minimally modified low-density lipoprotein (MM-LDL) for 24 hours caused a 2- to 3-fold increase of P-selectin protein, with little change in P-selectin surface expression. A 15-minute histamine treatment of cells exposed to MM-LDL caused a 50% to 100% increase in P-selectin surface expression compared with cells not treated with the lipoprotein. This increase resulted in a 2-fold increase in binding of leukocytes to the endothelium. Immunostaining of permeabilized HAECs after MM-LDL treatment also revealed a highly reproducible increase in intracellular P-selectin associated with rod-shaped structures, typical of Weibel-Palade bodies. Oxidized phospholipids were shown to be mainly responsible for the action of MM-LDL. This increased P-selectin expression was associated with MM-LDL-induced cAMP elevation. Like histamine, highly oxidized low-density lipoprotein, especially the oxidized fatty acids, caused immediate redistribution of P-selectin to the cell surface followed by reinternalization. Immunohistochemical staining showed that endothelial cells on human fatty streak lesions expressed increased levels of P-selectin compared with nonlesion areas. These studies suggest that P-selectin may play an important role in early recruitment of mononuclear cells to the subendothelium in human atherosclerosis and that oxidized lipoproteins may contribute to the increased expression of this molecule by increasing intracellular stores and causing redistribution to the cell surface.     

Expression of cell adhesion molecules and the appearance of adherent leukocytes on the left atrial endothelium with atrial fibrillation: rabbit experimental model

To assess the expression of cell adhesion molecules and the appearance of leukocytes adhering to the left atrial endothelium with atrial fibrillation (AF), 10 Japanese white rabbits were anesthetized and 3 pacing leads were placed in the right atrium. For the AF model, the right atrium was stimulated by electrical pacing (the stimulation frequency of each lead being adjusted to different intervals) for 8h while the control model was subjected to a sham operation without atrial stimulation. The left atrial appendage was excised from the heart and examined immunohistochemically. P-selectin staining of the endothelium in both models was linear and regional, and intracellular adhesion molecule-1 (ICAM-1) in the AF model was confined to leukocytes and endothelial cells with adherent leukocytes. The expression of P-selectin (p<0.05) and the appearance of positively ICAM-1 stained adherent leukocytes (p<0.05) were significantly greater in the AF model than in the control model. In conclusion, AF could regulate the expression of at least 2 critical adhesion molecules, P-selectin and ICAM-1, and the appearance of adherent leukocytes; suggesting that these molecules may play an important role in left atrial thrombus formation with AF. Although anticoagulant therapy has generally been carried out with warfarin in AF patients, neutralizing antibodies to cell adhesion molecules should be tried to prevent thromboembolic complications.